Thin film transistors (TFTs) are widely used in situations where conventional (single crystal) devices are not economical or practical. A typical example would be in the active matrix of a liquid crystal display. Typically, polysilicon is used for the active material to form the channel. To provide maximum resistivity this layer is undoped (intrinsic). It is deposited either above or below a metallic gate electrode with a layer of gate oxide interposed between them. In order to form source and drain regions in the amorphous silicon on opposing sides of the gate electrode, it has been standard practice in the art to make use of ion doping to accomplish this. This makes for a relatively expensive process which for certain classes of mass produced product could make the latter non-competitive. The present invention provides a method for doping the source and drain regions at lower cost and involving a lower energy budget.
In the course of searching for prior art, no teachings along the exact lines of the present invention were found. Several references of general interest were however encountered. For example, in U.S. Pat. No. 5,616,741, Young et al. form a field relief region through lateral diffusion of material from a conventionally formed (ion implanted) source region. Similarly, while Wu et al. (U.S. Pat. No. 5,612,235) use laser annealing to form a polysilicon layer, the source and drain regions are conventionally formed through ion implantation. Masao et al. (U.S. Pat. No. 4,609,407) show how a single crystal layer may be formed from a poly layer by annealing with a laser beam.